These studies are designed to further elucidate our understanding of the interactions between defense mechanisms of the lung and the important human pathogens Streptococcus pneumoniae, Klebsiella pneumoniae, Straphylococcus aureus, and Pseudomonas aeruginosa. As compared to treatment of existing infection, significant advances in the prevention of overt lung infection are dependent on a better understanding of these mechanisms. Additionally, the interaction of bacteria or bacterial products with lung defense mechanisms may be important primary or secondary factors in the genesis and progression of chronic lung diseases. We propose to systemically examine defense mechanisms that may be important in erradicating bacteria during the early interval after their inoculation into the lung and the interrelationships between these mechanisms. We shall utilize animal models to 1) determine the type of phagocyte primarily responsible for early lung defense; 2) determine the protection afforded against aerobic bacteria by type specific antibodies of the major immunoglobulin classes in serum and lung washings; 3) determine the course of appearance of specific antibodies of each immunoglobulin class in lung washings and serum after immunization by pulmonary and systemic routes; 4) determine the role of complement in lung defenses.